Orthogonal frequency division multiplexing (OFDM) and multiple-access orthogonal frequency division multiplexing (OFDMA) are two commonly used physical layer transmission schemes in wireless communications networks. Combining the multiple-input-multiple-output (MIMO) technique with OFDM or OFDMA greatly enhances the performance of a wireless communications network. The MIMO technique, which employs an array of antennas to transmit concurrently multiple signal streams to a mobile station (MS), improves the throughput and reliability of a wireless communications network.
The most commonly used MIMO methods are space-time coding (STC) and spatial multiplexing (SM). An STC method exploits spatial and temporal diversity to provide better reliability for a wireless communications network. By contrast, an SM method exploits spatial diversity to improve the throughput of a network. Several STC and SM methods have been developed for a network with a 2-antenna base transceiver station (BTS) employing the MIMO technique. However, there are no known STC or SM methods that can achieve the same level of performance when applied to a network with a BTS which has an antenna array of more than two antennas as in a network with a 2-antenna BTS.
A couple of issues occur when an STC or SM method is employed in a wireless communications network with a BTS which has an antenna array of more than two antennas. For example, it is necessary to reserve extra channel resources for non-overlapping, non-interfering pilot signals for each antenna in the antenna array. Since the allocation of pilot signals to every frequency must be repeated regularly, it consumes a lot of radio bandwidth and thus reduces the utilization of a wireless communications network. In addition, detecting signals transmitted from an antenna array of more than two antennas is more complicated than detecting signals from a two-antenna array, thereby making the signal acquisition process complex.
Currently, most wireless communications networks are designed in such a way that they function well when BTSs in these systems have two antennas and mobile stations (MSs) only have the ability to process MIMO signals transmitted from two antennas. When a BTS is upgraded to have more than two antennas, an MS must also be upgraded in order to benefit from receiving additional signals transmitted from additional antennas.
As such, what is desired is a system and method for creating a desired number of MIMO signal streams in a wireless communications network with a BTS that has an antenna array of more than two antennas. The desired number is less than the number of antennas in the antenna array.